1. Field of the Invention
The present invention is in the field of orienting a liquid crystalline substance relative to a substrate to produce a high contrast between various phases of the liquid crystalline substance. The method involves pretreating the surface of the substrate with an aqueous or organic solvent solution of a steroid and/or a porphyrin dyestuff.
2. Description of the Prior Art
The properties of liquid crystalline substances are utilized for displays and for thermography on the basis of their characteristic of providing an optically identifiable texture transformation.
Thermography, i.e., the measurement of temperature distributions on surfaces, is gaining increasing significance for the development and quality analysis of integrated circuits and other current-carrying electrical components. The presently most sensitive method for identification of "hot spots" which are locally limited, thermally active regions, is temperature measurement with liquid crystal layers in a polarization microscope. Liquid crystal layers are applied to the surface to be investigated. A study of the birefringence of the liquid crystal in the polarized light serves to determine the structures of the liquid crystal. Temperature differences within the surface to be investigated can be determined by structural modifications of the liquid crystal layer on the surface being investigated. Electrical properties within this surface can then be inferred in turn from the temperature differences within the surface to be investigated.
When a liquid crystal layer has been applied to the surface of an integrated circuit, the voltage at that location on the surface which is impinged by a laser beam can be inferred from the intensity of the reflection of the laser beam by means of the liquid crystal layer/surface.
The principle of a "hot spot" detector has been disclosed in PCT patent application No. WO 83/01989. This PCT application is based upon U.S. Ser. No. 326,224, U.S. Pat. No. 4,466,746, filed Dec. 1, 1981 in the names of Hancock et al. This publication refers to an ebulliometric hot spot detector wherein a semiconductor device is placed in a suitable liquid and the liquid pressure is decreased until boiling from sites on the device is observed. The liquid pressure is then increased until boiling ceases while the boiling sites on the device are observed. Small bubbles observed just before cessation of boiling pinpoint the hot spot locations. The pressure at which boiling ceases in a given liquid determines the hot spot temperature.
A uniform orientation of the molecules of liquid crystalline substances across larger surface areas is necessary for the exploitation of properties of such liquid crystalline substances. This orientation can be achieved by means of suitable methods for treating the surfaces that come into contact with the liquid crystalline substances.
Up to the present, surfaces that come in contact with the liquid crystalline substances have been treated by means of rubbing, chemical etching, coating with synthetic resins, organosilanes or amphipatic substances such as lecithin. For example, U.S. Pat. No. 3,728,008 discloses the chemisorption of a silane derivative on those surfaces of a liquid crystal display means that come in contact with the liquid crystalline substance. The chemisorbed silane derivative is intended to effect a clear display and to improve the contrast of the display.